The present invention relates generally to coating compositions, particularly clearcoat compositions, especially clearcoat compositions for refinish coatings.
The automotive industry has made extensive use of basecoat-clearcoat composite coatings for automotive body panels and components because such composites offer exceptional gloss, depth of color, distinctness of image, or special metallic effects. Refinish coatings, likewise, have moved toward two-layer basecoat-clearcoat systems for repairing original equipment coatings so that the refinish coating will have approximately the same appearance as the original coating.
Single-layer topcoats and the clearcoats of color-plus-clear composite coatings require an extremely high degree of clarity and gloss to achieve the desired visual effect. Such coatings require a low degree of visual aberrations at the surface of the coating in order to achieve the desired high gloss and high distinctness of image (DOI). Because defects are so noticeable in the smooth, glassy surfaces required for these coatings, they are especially susceptible to a phenomenon known as environmental etch. xe2x80x9cEnvironmental etchxe2x80x9d is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish of the coating that often cannot be rubbed out. It has been difficult to predict the degree of resistance to environmental etch that a high gloss topcoat or color-plus-clear composite coating will exhibit.
Traditional high solids enamels, in which a hydroxyl-functional acrylic polymer is crosslinked with an aminoplast resin, do not provide the desired level of resistance to environmental etch. While the ether linkages formed by aminoplast resin crosslinkers, for example a melamine-formaldehyde resin crosslinked with a hydroxyl-functional resin, are undesirable from the standpoint of resistance to environmental etch, aminoplast crosslinkers are desirable for other reasons, such as providing high solids coatings having excellent appearance that cure under moderate conditions.
Coating compositions containing carbamate-functional polymers and aminoplast resin curing agents have improved resistance to environmental etch. The improved resistance to environmental etch has increased the use of carbamate-functional polymers with aminoplast resins in original finish coating compositions. Coating compositions with carbamate-functional polymers, however, typically have an increase in regulated volatile organic compound emissions compared to the traditional high-solids enamels. Thus, it would be desirable to reduce the regulated volatile organic compound emissions in the coating compositions containing carbamate-functional polymers. This need to reduce regulated emissions is particularly acute for refinish coatings, which are applied by small businesses that cannot afford the elaborate and expensive high efficiency abatement equipment of the original equipment manufacturers.
Etzell, U.S. Pat. No. 4,546,046, incorporated herein by reference, describes epsilon-caprolactone modified acrylic polymers in coating compositions for increased flexibility so that the coating can be applied to flexible as well as rigid substrates. The epsilon-caprolactone-modified acrylic also provided higher solids compositions. The epsilon-caprolactone extended acrylic polymer described in the Etzell patent does not provide sufficient resistance to environmental etch, however, because the curing mechanism is again the reaction between an aminoplast crosslinker and hydroxyl functionality, including the terminal hydroxyl of the caprolactone extension, of the acrylic polymer.
Argypolous, U.S. Pat. No. 5,412,049, incorporated herein by reference, describes (meth)acrylate copolymers obtained by copolymerization of one or more hydroxyl-functional (meth)acrylate esters and one or more (meth)acrylate esters of hydroxyalkyl carbamates. The latter monomer may be prepared by different means to provide the ester structure, including direct esterification of (meth)acrylic acid with the hydroxyalkyl carbamate compound. The hydroxyl-functional monomer can be the product of reacting epsilon-caprolactone with a hydroxyalkyl acrylate. The copolymers are then crosslinked with a crosslinking agent reactive with the hydroxyl groups or the carbamate groups to provide a cured coating composition. The Argypolous reference does not mention preparation of a polymer having a hydroxyl group beta to a carbamate group.
Ohrbom, U.S. Pat. No. 6,106,951, incorporated herein by reference, describes a coating composition containing a carbamate-functional resin, including a beta-hydroxy carbamate-functional resin, in which non-carbamate groups that are reactive with an aminoplast crosslinker, particularly hydroxyl groups, are converted to non-reactive moieties to prevent formation of ether linkages when the acrylic is cured with the aminoplast crosslinker. Because thehydroxyl groups no longer can react with the aminoplast crosslinker to form therelatively weak ether linkages, the resistance of the cured coating to environmental etch is increased. The Ohrbom reference does not describe a modification to reduce the viscosity of the resin.
It would be desirable to produce a lower viscosity, beta-hydroxy carbamate-functional copolymer without increasing the hydroxyl groups by adding modified hydroxyl monomers that produce undesirable ether linkages during crosslinking, while maintaining the higher resistance to environmental etch of carbamate-functional resins.
The present invention provides a coating composition comprising a carbamate-functional acrylic polymer having a monomer unit with an epsilon-caprolactone moiety beta to a carbamate group. In one aspect of the invention, the monomeric unit including the epsilon caprolactone moiety and carbamate group includes the following structure: 
in which R is hydrogen or alkyl, preferably alkyl of from 1 to about 8 carbons, more preferably alkyl of from 1 to about 4 carbons; n is, on average, from about 0.5 up to about 10, preferably from about 0.5 to about 6, more preferably from about 1 to about 4, and still more preferably from about 1 to about 3; X is hydroxyl, or, preferably, does not have functionality that reacts with the crosslinker when the coating composition is cured; one of Rxe2x80x2 and Rxe2x80x3 is H or alkyl of up to 4 carbon atoms, preferably H, and the other of Rxe2x80x2 and Rxe2x80x3 represents a divalent radical connecting the structure to the remainder of the monomeric unit.
While not wishing to be bound by theory, it is believed that the epsilon caprolactone moiety of the present invention is particularly effective in reducing the viscosity of the polymer because of its proximity to the carbamate group, being beta to the carbamate group.
In one embodiment of the invention, the epsilon-caprolactone moiety, which is formed with a terminal hydroxyl group, is further reacted to convert the hydroxyl group into a group that is not reactive with the crosslinker, particularly not reactive with an aminoplast crosslinker, under the crosslinking conditions. Even though the epsilon moiety may not then participate in the crosslinking reaction, the benefit of reduced viscosity is retained, while at the same time no weak ether linkages are formed by reaction of a hydroxyl group with the aminoplast crosslinker. In particular, X in the structure above may have no active hydrogens.
The polymer of the present invention having an epsilon-caprolactone moiety and a carbamate group on the same monomeric unit may be produced in two different ways. The first method of producing the polymer of the present invention includes a step of reacting a carbamate-functional polymer having beta-hydroxyl groups with epsilon-caprolactone to produce an epsilon-caprolactone moiety beta to the carbamate group on the same monomeric unit. The second method of producing the polymer having an epsilon caprolactone moiety beta to a carbamate group includes steps of reacting a beta-hydroxy carbamate monomer with epsilon-caprolactone to produce a monomer with a caprolactone moiety beta to a carbamate group and then polymerizing the monomer having epsilon-caprolactone moiety beta to the carbamate group to produce the polymer of the invention.
The present invention further provides a coating composition containing the polymer having an epsilon-caprolactone moiety beta to carbamate group and an article having a substrate upon which there is a cured coating derived from the coating composition according to the invention.